Some Physical-Chemical Fundamentals

Abstract

Melting Point Determination. In the last chapter the matter of crystal growth was discussed from a geometrical and structural viewpoint. It is now necessary to examine the question, “When does a crystal crystallize from a melt?”. The obvious answer is that it occurs when a fluid is cooled a bit below the melting temperature (melting point) which prevails at some definite pressure. In order to determine the exact melting point in those cases where it is not possible to directly observe the melting process, it is possible to take advantage of the fact that heat is liberated during crystallization, the so-called heat of melting or crystallization. If a melt, for example, of a simple metal, is allowed to cool slowly, and the temperature measured as a function of time, it is observed that in spite of the cooling, a definite temperature is reached and the melt remains at this temperature for a short period of time. This is the precise melting point. For this short period of time the drop in temperature is compensated for by the liberated heat of melting, or crystallization. Only when the melt has completely crystallized does the temperature decrease further. Conversely, it can be observed upon heating crystals that at the melting point, because of absorption of the heat of melting, a temperature increase is delayed until everything is fluid. In a like manner boiling points can be determined through similar effects of the heat of vaporization. Changes in structural modification can be noted likewise by similar effects due to liberation of the heat of transformation.